In the development of transmitters for wireless devices, envelope branch type transmitters are considered as an architectural concept for improving transmitter efficiency. In such envelope branch type transmitters, a signal portion (hereinafter referred to as signal) and an envelope portion (hereinafter referred to as envelope) are derived from an input signal, and the signal and the envelope are separately supplied to a power amplifier via different branches. The power amplifier generates a transmit signal on the basis of the signal supplied on a signal branch and the envelope supplied on an envelope branch, wherein a supply voltage of the power amplifier is modulated in accordance with the envelope. Thereby, an increased supply voltage to the power amplifier during signal peaks of the input signal is enabled, which improves signal quality and output power with the same or even lower power consumption as compared with a conventional transmitter architecture with constant power amplifier supply voltage.
In such envelope branch type transmitters, a delay mismatch between the signal branch and the envelope branch causes distortion. Such distortion basically results from (interaction of the delay mismatch with) various nonlinear effects of the power amplifier, including one or more weak nonlinear effects such as nonlinear transconductance in transistors, strong nonlinear effects, such as saturation in transistors and clipping, and frequency-dependent nonlinear effects such as the reactance of a bias line interacting with one of the above weak or strong nonlinear effects. For example, a large delay mismatch may cause insufficient supply voltage at the power amplifier during input signal peaks, causing the peaks to be cut off.
Such nonlinear effects in the power amplifier thus cause unwanted spectral components at the output of the power amplifier, i.e. in the transmit signal, that may interfere with radio transmissions in the transmit and/or receive directions of a wireless device. Accordingly, the caused unwanted spectral components result in interference which may occur in the transmit band of the transmitter, either as adjacent channel or in-channel interference, or in the receive band of a receiver within the same transceiver as the transmitter.
Accordingly, the delay mismatch between the signal branch and the envelope branch of an envelope branch type transmitter needs to be balanced or compensated with high accuracy (for example in the order of nanoseconds) so as to reduce interference. However, a single delay balance (or compensation) value typically cannot be found which is capable of sufficiently reducing interference both in the transmit band of the transmitter, i.e. adjacent channel and in-channel interference, and in the receive band of a receiver within the same transceiver as the transmitter. That is to say, when applying a single fixed delay balance (or compensation) value, potentially significant interference remains in at least some frequency ranges, which could, in certain cases, only be suppressed by using more expensive (duplex) filters.
Thus, there is a need to improve interference reduction for an envelope branch type transmitter.